A San Jose middle school was evacuated for about three hours Friday when a staff member found an backpack containing a bottle labeled “Nitroglycerin” — police said.

Police were called to Bridges Academy at 1702 McLaughlin Ave. at 8:21 a.m. to investigate the suspicious liquid found in the backpack. Officers ordered the school evacuated, set up a perimeter around the campus and blocked off traffic in both directions of McLaughlin Avenue.

A bomb squad and hazardous materials crew responded to investigate.

“The bomb squad removed the suspicious liquid and will test it before disposing of it,” said Sgt. Enrique Garcia of the San Jose Police Department.

It was not immediately clear if the liquid was actually nitroglycerin, a highly explosive substance.

Officers conducted a complete sweep of the campus before clearing the school to reopen about noon and allowing traffic back on McLaughlin Avenue.